In the processing of cellulose material, for example, wood or recycled paper, liquid streams are produced that contain cellulose fibers or other impurities that are typically undesirable in the process in which the liquid is used. For example, cellulose fibers must be minimized or removed from spent cooking liquors before the liquors are forwarded to evaporators in a chemical recovery system. Otherwise, the cellulose fibers are minimized or removed from effluents of pulp washers or pulp presses used in processing of the virgin pulp or recycled pulp before the effluents can be used elsewhere.
Cellulose fibers and other undesirable material can be removed from liquid process streams by a filtering device. One type of filtering device that is typically used is a pressurized screening device. These filtering pressurized screening devices are similar to pulp-processing pressurized screening devices that isolate or xe2x80x9cscreenxe2x80x9d large uncooked pieces of cellulose material, such as pins and shives, from cellulose pulp streams.
Unlike pulp-processing pressure screening devices, pressurized screening filtering devices (typically referred to as xe2x80x9cfiber filtersxe2x80x9d) filter undesirable material from liquid streams. Liquid streams typically have a much lower content of cellulose material than do the pulp streams fed to pulp screening devices. Pressurized pulp screening devices are typically fed cellulose pulp slurries having about 1 to 5% by weight (that is, about 1 to 5% consistency). In contrast, fiber filters are typically fed liquid streams having a fiber content of less than about 1% consistency, and preferably less than about 0.1% consistency, or even less than about 0.05%. For example, the liquid streams treated with fiber filters typically contain fiber in the range of 50 to 250 parts per million (ppm), which corresponds to a consistency of approximately about 0.005 to 0.025%.
Pressurized screen fiber filters typically contain perforated cylindrical screen baskets through which the fiber containing liquid stream is passed to remove the fibers and other materials. One typical fiber filter is the MODUScreen(trademark) FF Pressure Filter provided by Andritz-Ahlstrom, Inc., of Glens Falls N.Y. This pressurized fiber filter screening device consists of a stationary cylindrical screen cylinder mounted about a cylindrical rotor. The stationary perforated cylinder contains circular through-holes, having an exemplary 0.2 mm diameter. However, the cylinder holes may be shaped as parallel bars or machined slots. In operation of the fiber filter, the liquid to be filtered flows from inside of a screen basket in the fiber filter, then radially outward through the screen where the undesirable material particles (for example, cellulose fibers), are prevented from passing through the screen perforations. These undesirable materials captured by the screen basket are passed out through a rejects outlet in the fiber filter and forwarded for further processing.
The cylindrical rotor of the fiber filter is provided with a series of uniformly spaced protrusions or xe2x80x9cbumpsxe2x80x9d. The leading edge of the bumps acts as a hydrofoil to impart a hydraulic pulse to the liquid flowing along the inside of the basket. The trailing edge of the bumps creates a fluid pressure drop which momentarily draws the liquid back through the holes to dislodge any material that may be stuck in the holes so as to minimize screen pluggage. This type of fiber filter has been effective in minimizing or removing cellulose fiber and other undesirable materials from liquid streams in and around pulp and paper mills.
Sometimes the liquid stream introduced to these filtering devices contains undesirable particles that cannot be easily isolated with conventional pressurized screen fiber filters. In particular, smaller particles are not easily isolated and tend to pass through the screen. Particles smaller in diameter than the diameter of the holes in the screen cylinder especially tend to pass through the screen holes. For example, particles smaller than the 0.2 mm diameter (about 0.008 inches) of the holes in the Andritz-Ahlstrom MODUScreen FF Filter are not easily isolated and tend to pass through the screen.
One method of capturing smaller particles is to reduce the size of the screen perforations to 0.15 or 0.1 mm (that is, 0.006 or 0.004 inches), for example. However, smaller diameter screen holes can be much more expensive and difficult to fabricate using existing drilling or cutting technology. Another alternative is to wrap the screen cylinder with a fine cloth, fabric, or wire screen having a tight weave to capture the smaller particles. Such fabrics or wire screen are prone to tearing and other damage from the loading and high speeds under which present fiber filters are operated. Neither of these alternatives are particularly feasible or economical under the present state of the technology.
According to the present invention, a fine screening medium is provided to the screen cylinder of a pressurized screen fiber filtering device. This fine screening medium is formed by adding at least some cellulose fibrous material to the filtering device so as to form a web or mat of cellulose material on the existing perforated surface. This web of cellulose fibers acts as a fine screening medium to filter small cellulose fibers and other undesirable material.
An embodiment of this invention is a process for removing solid particles from a liquid stream in a pulp or paper mill having a filtering device. The filtering device includes an inlet for liquid having at least some undesirable materials, an outlet for treated liquid having reduced concentration of undesirable material, an outlet for undesirable material, and a perforated screen element. A filter forming process is first used that comprises the steps of: (a) introducing a first liquid having an undesirable material content to the filter inlet; (b) introducing a second liquid having at least some comminuted fibrous material to the filter inlet to form a mixture of the first and second liquids; (c) passing the mixture of the first and second liquids through the screen element to produce a third liquid having little or no undesirable material; and (d) discharging the third liquid from the filtering device. As a result of this filter forming process, at least some of the comminuted cellulosic fibrous material introduced with the second liquid is retained on the screen element to form a permeable mat of cellulose material. The mat of cellulose material acts as a filtering medium for the undesirable material in the first liquid. The mat is retained on the screen element and is used to filter liquid streams flowing through the filter.
The filtering device is preferably a pressurized device having a cylindrical screen element. For example, the filtering device with cellulose mat may be a modified MODUScreen FF Pressure Filter provided by Andritz-Ahlstrom. Preferably, the second liquid containing at least some comminuted cellulosic fibrous material (for example, hardwood or softwood fibers), is mixed with the first liquid prior to being introduced to the filter inlet. The second stream containing the cellulose fibers may be introduced continuously to the first stream or may be introduced intermittently. For example, the cellulose steam may be automatically introduced in response to the state of the cellulose web or mat produced on the surface of the filter element, for example, as indicated by an electrical signal corresponding to the pressure drop across the filter element.
The invention may also include a system for treating a liquid stream in a pulp and paper mill to remove undesirable material from the liquid stream. The system comprises a filtering device having a perforated screen element having a filtering surface, an inlet, and a filtered liquid outlet; means for introducing a liquid containing undesirable material to the inlet; means for introducing a liquid containing at least some comminuted cellulosic fibrous material to the inlet; and means for forming a bed of comminuted cellulosic fibrous material on the filtering surface of the screen element to provide a finer screening medium than the screen element without the comminuted cellulosic fibrous material present. The screen element is preferably a cylindrical screen element. The cylindrical screen element may be stationary or it may rotate. The filtering surface of the cylindrical screen element may be an external surface or an internal surface, that is, the liquid may pass radially inward or radially outward through the cylindrical screen element.